For field choppers, the distance between the chopping knives and the shearbar is an important parameter for the quality of the cut and the power applied for cutting. Such shearbars are usually adjustable by electromotors relative to the chopping drum. A series of devices has been proposed (DE 41 34 957 A, DE 43 35 786 A, EP 0 706 752 A), which include a knocking sensor that is attached to the shearbar and whose output signal is a function of the distance between the shearbar and the chopping knives. For these devices, the output signal of the knocking sensor is supplied to a low-pass filter and the amplitude is used to decide whether or not the shearbar is sufficiently close to the chopping knives. It is considered a disadvantage that selection of the amplitude for which the distance is considered to be correct is very critical. If it is selected to be too small, the distance is too large; if it is selected to be too large, collisions between the chopping knife and the shearbar are possible. However, the amplitude of the knocking sensor signal also depends on other conditions, such as the state of the sensor, etc.
Furthermore, to measure the distance between the shearbar and the chopping knives, magnetic sensors are known that include a permanent magnet connected to the shearbar and an induction coil in which an electromotive force (EMF) is induced when the chopping knife brushes past. This electromotive force is amplified and then detected. In such a device, which is described in EP 0 943 888 A, the induced voltages are subjected to frequency analysis. The ratio of the high-frequency portions of the signal spectrum to the low-frequency portions is taken. The quotient determined in this way provides information on the distance between the shearbar and the chopping knives. The requirement of using a permanent magnet, which can disturb the functioning of a metal detector arranged close to the shearbar, is considered a disadvantage.
In DE 199 03 153 C it is proposed to measure the forces exerted by the chopping knife on the shearbar, and to determine the sharpness of the chopping knife with reference to the measured values.
In the article “Messverfahren zur Feststellung des Schärfezustandes der Paarung Häckselmesser/Gegenschneide eines Feldhäckslers [Measurement method for determining the sharpness condition of the chopping knife/shearbar pair of a field chopper]” by A. Heinrich and G. Bernhardt, published in Conference: Agricultural Engineering, meeting in Halle/Saale [Germany], Oct. 10-11, 2002 (VDI-MEG) on pages 325-330, another method for determining the sharpness of chopping knives is proposed. The acceleration of the shearbar is detected and input to a frequency analysis. Using the harmonic wave spectrum, it can be determined whether the chopping knives are still sufficiently sharp or not.
The two references previously mentioned present possibilities for determining the sharpness of chopping knives, but they give no information on measuring the distance between shearbars and chopping knives.
The invention is based on the problem of disclosing a device, improved relative to the prior art, for determining the distance between a shearbar and chopping knives that can move relative to the shearbar.